Aqueous sols of oxides of certain metals are known, but it is already more difficult to find descriptions of organic sols. Only sols, or colloidal solutions, of certain metals are described; with regard to sols of mixtures of metals, if they are described as such, they correspond to mixtures of metals only insofar as they involve an impurity (or impurities) caused by the precipitation of a main element.
Moreover, organic sols are mostly prepared from aqueous sols, but the particles of the aqueous sols are generally poorly adapted for extraction in an organic solvent. In order to obtain organic sols, it is therefore necessary to select the aqueous sols from which the organic sols will be made and often to provide a post-treatment for the purpose of rendering them extractable.
Even the preparation of aqueous sols is often difficult and it is necessary to find techniques which allow the production of individual particles having good dispersibility in water (the sol constituting merely a kind of dispersion).
Furthermore, for many uses, these sols must be stable and withstand various external conditions (temperature, stirring, etc.).
Thus the sols may form catalysts, or sources of catalysts, in organic liquid phase. The solo of catalysts may be used for drying paint (in general, crosslinking) and for catalysing the combustion of fuels, in particular for eliminating or greatly reducing the pollution by diesel engines using the technique described below.
The use as an additive for diesel engines constitutes a good example of the constraints with which such a sol must comply.
In the combustion of diesel fuel in a diesel engine, the carbonaceous products have a tendency to form soot, which is reputed both to be harmful both to the environment and to one's health. Techniques have been sought for a long time which allow a reduction in the emission of these carbonaceous particles which, in the rest of the description, will be referred to by the term "soot".
Many solutions have been proposed for reducing these carbonaceous emissions.
However, exhaust circuits are being increasingly fitted with a filter capable of stopping all, or a very high proportion (at least 80% by mass), of the carbonaceous particles generated by the combustion of various fuels.
This technique is, however, limited by the storage capacity of the filter, which either has to be emptied or the soot contained therein has to be burnt off. This so-called regeneration operation is extremely expensive to provide and to implement. One of the solutions most commonly proposed is the combustion of this soot, which combustion is brought about, intermittently, either by electrical heating or by the use of a fossil igniter fuel.
However, this technique has many drawbacks, not the least of which is the risk of thermal shock leading to fracture or cracking of a ceramic filter or to melting of a metal filter.
Another solution which would be satisfactory consists in introducing, into the soot, catalysts which allow frequent spontaneous combustion of the soot collected in the filter. To do this, it is necessary for this soot to have a spontaneous ignition temperature low enough to be frequently reached during normal running of the engine.